Semiconductor light emitting devices such as light emitting diodes (LEDs) are among the most efficient light sources currently available. Material systems currently of interest in the manufacture of high brightness LEDs capable of operation across the visible spectrum include group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials; and binary, ternary, and quaternary alloys of gallium, aluminum, indium, arsenic, and phosphorus. Often III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates and III-phosphide devices are epitaxially grown on gallium arsenide by metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. Often, an n-type region is deposited on the substrate, then a light emitting or active region is deposited on the n-type region, then a p-type region is deposited on the active region. The order of the layers may be reversed such that the p-type region is adjacent to the substrate.
One promising use of semiconductor light emitting devices is for backlights for general illumination and display devices such as liquid crystal displays (LCDs). LCDs are commonly used in cellular phones, personal digital assistants (PDAs), portable music players, laptop computers, desktop monitors, and television applications. One embodiment of the present invention deals with a color or monochrome, transmissive LCD that requires backlighting, where the backlight may use one or more LEDs. LEDs are distinguished from laser diodes in that the LEDs emit incoherent light.
The backlight illustrated in FIG. 1 is described in U.S. Pat. No. 7,052,152, which is incorporated herein by reference. An array of LEDs 24 is placed on the rear panel of the backlight 26. The back plane 48 and sidewalls 46 of the backlight 26 are covered with highly reflective materials. LCD panel 14 is placed in front of backlight 26. LCD panel 14 may be a conventional LCD, having a first polarizing filter, a thin film transistor array for developing an electric field across selected areas of the liquid crystal layer, a liquid crystal layer, an RGB color filter array, and a second polarizing filter. The color filter array has red, green and blue subpixels. Between the LCD panel 14 and the backlight 26, additional films can be used, such as a brightness enhancement film (BEF) or polarization recovery film (DBEF).
A color converting phosphor layer 39 is disposed on a cover plate 40. The phosphor layer 39 is a uniform layer, consisting of one or more different types of phosphor. Preferably, a green and a red phosphor are used, but a yellow phosphor (YAG) could be used as well. This layer 39 can be applied by spray painting, screen-printing, or electrophoretic deposition, or might be a film with uniform density of particles or a luminescent dye distributed throughout the film. As an alternative to a single phosphor layer, one type of phosphor is applied to cover plate 40, while another phosphor is applied to the rear panel 48 of the backlight. The phosphor on rear panel 48 may be applied not as a uniform coating, but as a dot pattern.